1. Field of the Invention
The present invention relates to a method and a device for seismic prospecting using a source of seismic vibrations at the bottom of a well.
2. Description of the Prior Art
According to one technique in common use, a well is drilled by a powered drive table at the surface driving a drill string composed of a set of interconnected pipes will a drilling tool attached at its lower end.
Drilling tools of this type produce powerful seismic waves, and the idea of using these waves to measure the rock traversed by the tool during drilling, particularly in seismic prospecting (these operations are known as MWD), with a view to decreasing oil prospecting costs has already been conceived, since the wells are not out of action while measuring is being conducted.
Various embodiments in this field are described in French Patent 1,584,915 and U.S. Pat. Nos. 4,965,774, 4,964,087, 4,926,391, 4,718,048, 4,862,423, 4,675,852, etc.
However, this type of operation is difficult to implement for numerous reasons. One of them is that the characteristic of the signals emitted and the emission spectrum of the source are generally poorly known because of the great distance between the emission location and the surface station. Another is that the vibrations are usually picked up by accelerometers placed in contact with the tubular drill string at the head thereof, after being propagated mechanically over its entire length. Under these conditions, the transmitted signals are strongly altered by complex resonance and absorption phenomena along the string and depend on the structure of the surface installation. Since the transfer function of the entire assembly is poorly known, there is no strict reference for processing the seismic signals.
A signal truly representative of the vibrations emitted by the tool can be obtained if the accelerometer is attached to the bottom of the drill string provided it is possible to transmit the measurements to the surface at a sufficient rate. It is possible to do this using modulated waves such that the sound waves propagate in the fluid contained in the well provided the data transmission rate remains relatively slow.
For applications where the data transmission rate is high, as is the case in prospecting operations, very large down-hole data storage devices can be used, but they must be brought up to the surface regularly by withdrawing the drill string or pulling up the cable in order to dump their contents.
Another technique available for high-speed transmission of signals picked up by one or more down-hole accelerometers is described for example in the Assignee's French 2,688,026 and 2,700,018. It consists essentially of using a drill string of which at least a part is equipped with a line for transmitting reference signals from the down-hole sensors to the surface station.
Difficulties of interpretation also occur due to the fact that the vibrations are not emitted solely at the drilling tool. It has been found that the drill string itself acts as a source of secondary vibration. This is also the case in a surface drilling platform that transmits mechanical energy to the rock through its feet, the transmitted energy being propagated along the drill string and acting as another source of secondary vibration.
It is known and easily verified that the guided waves propagated by a drill string are less quickly attenuated, as a function of the distance from the source, than the volume waves propagated in the formation. Since the guided waves radiate in the formation and create parasitic secondary vibrations, it is justifiable to install appropriate drilling assembly elements to attenuate these effects.
The above-mentioned French Patent 2,700,018 shows that absorbing elements, and possibly resonating elements or acoustic discontinuity elements favoring resonance in the seismic frequently range, can be interposed in the drill string to damp the vibrations propagating directing to the surface along the drill string.
The use of down-hole sensors, possibly associated with in situ data storage devices, is not always possible or advisable for the following reasons:
the down-hole measuring equipment is not designed to operate at the high temperatures prevailing at the bottom of the borehole in geothermal applications or at great depths, and PA1 the depth at which the down-hole equipment must operate renders inoperable the wave transmission devices designed to transmit the measurement signals to the surface recording and control station. PA1 By shortening the distance between the sensors and the surface acquisition and recording station, signal transmission is simplified and facilitated. PA1 In all applications where the well bottom temperature is very high: great depth, geothermal operation, etc., it is easier to shield the sensors and possibly the in situ data storage device; PA1 It is always possible, when nonphysical wave transmission device are being used for transmitting the reference signals to the surface, to locate them with the sensors fairly high up on the drill string and thus beyond the maximum wave propagation distance in the formation.
Other than the above-noted drawbacks, use of a sensor at the top of the drill string often proves impossible in directional wells because of the multiple reflections that occur along the part of the drill string below the bend, which often distort still further the reference signals used to correlate the recordings.